The present invention relates to a signal equalising arrangement and to a method of equalising a received data signal and has particular but not exclusive application in the transmission of data or digitised signals over radio channels. For convenience of description in the present specification digitised signals will be referred to as data.
The problem of intersymbol interference resulting from transmission of data on dispersive communication channels is known and in order to estimate correctly the data in the received channel, this signal is applied to a signal equaliser. Known signal equalisers are linear equalisers (LEs) which essentially are feedforward digital filters, decision feedback equalisers (DFEs) which essentially are a combination of a feedforward filter and a feedback (or recursive) filter including a decision stage, and Viterbi equalisers in which a received signal is compared with signatures stored in a memory bank and the best match is obtained. The present invention is particularly applicable to DFEs.
When operating a DFE, data is normally processed in the order in which it is received. The tap weights used in the feedforward and feedback filter sections are determined in dependence on the channel impulse response (CIR). The CIR may be substantially constant for communication channels such as telephone cables but may be variable in mobile (including portable) radio applications in which it may be necessary to recalculate the tap weights to suit the changing CIR. Another factor which has an influence on the performance of a DFE, expressed in terms of signal to noise ratio (SNR) versus bit error rate (BER), is the choice of the reference tap position.
In the present specification the expression reference tap and reference tap position are to be understood as meaning a synchronisation marker between the transmitter and the receiver. The reference tap position serves to compensate for the time delays occurring between the instant of transmission of a currently detected symbol and the instant a decision is made to estimate the symbols.
K. H. Mueller and M. Muller in an article "Timing Recovery in Digital Synchronous Data Receivers" IEEE Transactions on Communications Vol. Com.-24, No. 5, May 1976 pages 516 to 531 reported that in their opinion the energy in received signal samples can be maximised with respect to noise by adjusting a receiver clock until the clocking point is aligned with the peak of the impulse response of the channel. Consequently it has been customary when operating DFE's on telephone channels to synchronise the reference tap of the DFE to the peak of the impulse response.
H. Sari in an article entitled "Baseband Equaliser Performance in the Presence of Selective Fading" published by the IEEE Global Telecommunications Conference Proceedings 1983 pages 111 to 117 discusses LEs and DFEs and mentions that tap-gain optimisation in baseband equalisers is usually carried out by fixing the position of the reference tap (RTP) and then minimising the output mean square error (MSE) for that fixed RTP. Sari shows that by making the position of the reference tap adaptive, a considerable performance improvement can be achieved in both equaliser structures and especially in DFEs. In order to achieve RTP adaption Sari proposes the use of a second (or slave) equaliser for estimating the optimum RTP and transferring it to the main equaliser. At the commencement of operation the main equaliser will start with the centre tap reference, because it is considered to be the optimum position of the absence of fading. The slave equaliser will periodically try the N reference tap positions and for each RTP the estimated output MSE will be compared to that obtained with the main equaliser and every time a smaller MSE is obtained with the slave equaliser, the position of the reference tap of the latter as well as its tap-gain values will be transferred to the main equaliser.
It is also known to align the reference tap position with the location in the feedforward filter, which may be embodied as a tapped delay line or a shift register, which contains the last, that is the most recently received, acceptable sample of a current symbol. By an acceptable sample in the present specification is meant a sample which is above the noise floor.
An improved performance of a DFE has been obtained by aligning the reference tap position with the location in the feedforward filter which contains the first acceptable sample, that is the longest held sample, of a current symbol.
It has been found that on occasions the performance of the DFE expressed as SNR versus BER is several dB below the theoretical optimum. Accordingly it is an object of the present invention to improve the performance of a DFE.